It is known that a vesicle can be formed via self-assembly of polymer molecules of which the primary structure has been have been controlled precisely. Such a vesicle is applicable to various molecular designs, and can serve a new function beyond the properties of the original polymers. Accordingly, the vesicle is being considered for use as a carrier for a drug delivery system (DDS) or as a biomaterial or functional material.
Patent Document 1 (JP-H08-188541A, of which the inventors overlap with the present inventors) discloses a drug carrier in the form of an electrostatically-united polymeric micelle formed via self-assembly of molecules of a block copolymer having an uncharged segment and a charged segment.
Non-Patent Document 1 (Schlaad H. et al., Macromolecules, 2003, 36 (5), 1417-1420) discloses a vesicle referred to as “polymersome”, which is formed via self-assembly of molecules of a first block copolymer having poly(1,2-butadiene) block and poly(cesium methacrylate) block and a second block copolymer having polystyrene block and poly(l-methyl-4-vinylpyridinium iodide) block.
Patent Document 2 (WO2006/118260A, of which the inventors overlap with the present inventors) discloses a vesicle formed via self-assembly of a first block copolymer having an uncharged hydrophilic segment and a cationic segment (e.g., PEG-polycation) with a second block copolymer having an uncharged hydrophilic segment and an anionic segment (e.g., PEG-polyanion).
Non-Patent Document 2 (Anraku Y. et al., J. Am. Chem. Soc., 2010, 132 (5), 1631-1636, of which the inventors overlap with the present inventors) discloses a vesicle formed via self-assembly of a block copolymer having an uncharged hydrophilic segment and a charged segment (e.g., PEG-polycation) and a copolymer charged oppositely to the charged segment of the block copolymer (e.g., polyanion).
It is contemplated that such vesicles formed via self-assembly of polymers as mentioned above can encapsulate and carry various substances within their cavities for desired applications (for overview, see, e.g., Non-Patent Document 3: H. Nyin et al. Soft Matter, 2006, 2, 940-949; and Non-Patent Document 4: “Liposome: New Developments in Applications”, supervised by Kazunari AKIYOSHI et al., NTS Inc., 2005).
A typical process of producing a vesicle encapsulating a substance within its cavity (hereinafter also referred to as “substance-encapsulating vesicle”) includes mixing a substance to be encapsulated (hereinafter also referred to as “encapsulation-target substance”) with membrane component polymers or a preformed polymer membrane to cause formation of a polymer vesicle via self-assembly simultaneously with enclosure of the substance into the vesicle cavity (hereinafter also referred to as “simultaneous mixing method”). Examples include: emulsion method (see, e.g., Non-Patent Document 5: F. Szoka, Jr et al., Proc. Natl. Acad. Sci. USA, 1978 75 (9) 4194-4198); and instillation method using organic solution of lipids (see, e.g., Non-Patent Document 6: Batzri, S. et al., Biochim. Biophys Acta 1973, 298, 1015-1019).
However, the simultaneous mixing method has a drawback that presence of the encapsulation-target substance may affect vesicle formation process via self-assembly, thereby preventing formation of a vesicle or, even if not, enclosure of the substance into the vesicle cavity. Another problem involved in this method is that it often requires use of organic solvent which is detrimental to membrane formation, rendering the process complicated and causing damage to the encapsulation-target substance due to the organic solvent. This method has still another drawback that it is difficult to form vesicles having uniform particle size and structure unless carrying out an additional step, which is likely to render the process complicated. Thus, this method lacks versatility, and is not practical as a means for producing various kinds of substance-encapsulating vesicles.
On the other hand, as a general method of producing a particle encapsulating a substance, there is a method in which an encapsulation-target substance is introduced into the cavity of an existing vacant particle such that the substance is enclosed and carried by the particle (hereinafter also referred to as “post-carrying method”) (see, e.g., Non-Patent Document 7: W. Tong et al. J. Phys. Chem. B, 2005, 109, 13159-13165). This method could be an option for producing substance-encapsulating vesicles.
However, application of the post-carrying method to vesicles would require any additional means to introduce an encapsulation-target substance beyond the membrane of a vacant vesicle into the vesicle cavity. A conceivable method includes: making the vacant vesicle swell to relax the membrane; penetrating the encapsulation-target substance into the cavity through cleavage which has occurred on the relaxed membrane; and contracting the membrane to prevent release of the encapsulation-target substance. Another conceivable method includes: opening pores on the membrane of the vacant vesicle; introducing the encapsulation-target substance into the cavity through the pores; and closing the pores to prevent release of the encapsulation-target substance. However, these methods are cumbersome and complicated, too disadvantageous to be put into practical use. In addition, the particle size and the structure of the existing vacant vesicle would probably be disturbed during the process of enclosure and carriage of the encapsulation-target substance. Accordingly, these methods have been considered as being far from practical.
Another published method for lipid bilayer membrane vesicles such as liposomes includes integrating a channel protein into the lipid bilayer membrane (see, e.g., Non-Patent Document 8: Ranquin A, Versees W, Miere W, Steyaert J, Gelder P V., “Therapeutic Nanoreactors: Combining Chemistry and Biology in a Novel Triblock Copolymer”, Drug Delivery System, Nano Lett., 2005, 5:2220-4). However, this method is not practical either, since the process is cumbersome and complicated and lacks versatility.
Under such circumstance, the present inventors filed a patent application (Patent Document 3: WO2011/145745A) based on a surprising finding that a vacant vesicle formed of a membrane containing a first polymer, which is a block copolymer having an uncharged hydrophilic segment and a first charged segment, and a second polymer, which has a second charged segment oppositely charged to the first charged segment, wherein the membrane defines a cavity, can be used for producing a substance-encapsulating vesicle efficiently with ease by a method including mixing a vacant vesicle in aqueous medium in the presence of a target substance to be encapsulated (which method corresponds to a “post-carrying method” mentioned above), whereby the target substance is encapsulated into the vesicle cavity (inner aqueous phase) via self-assembly of the first and second polymers.